In recent years, many federal judges have voted to strike down bans on same-sex marriage, in part because no one has defended them well. This month, however, Judge Jeffrey Sutton, of the U.S. Court of Appeals for the Sixth Circuit, produced the most powerful defense to date -- one that will give the Supreme Court a serious test.

Judge Sutton acknowledged that “the question is not whether American law will allow gay couples to marry; it is when and how that will happen.” Nor did he lament what he saw as history’s arc. Instead he argued that, for federal courts, the only question is: Who decides? His answer: not judges, but the democratic process.

To defend this conclusion, Sutton noted that, in general, federal courts must uphold state laws unless they lack any “rational basis.” This test is meant to be highly deferential. If judges can come up with any plausible reason for a law, they must uphold it, even if that reason is not what motivated those who supported it.

Here’s where Sutton made a strong plea for judicial modesty. Noting that, until recently, marriage had been widely restricted to opposite-sex couples, he said, a “dose of humility makes us hesitate to condemn as unconstitutionally irrational a view of marriage shared not long ago by every society in the world.”

Sutton identified two justifications for that time-honored view. First, a central goal of the institution of marriage has been to encourage opposite-sex couples to enter into lasting relationships in order to protect children. Stable family units could be deemed valuable “for the planned and unplanned creation of children.” Of course, marriage has many other purposes, but as Sutton emphasized, the issue is one of constitutional law rather than policy-making, and rational basis review is a low bar.

Second, a state might rationally “wait and see" before changing a norm that society has endorsed for decades. It is not irrational, in other words, for state officials to see what happens in those states where the definition of marriage has been broadened.

Judge Sutton acknowledged that the Supreme Court has sometimes spoken of a “fundamental right to marry,” most famously in striking down bans on interracial marriage. But in his view, consistent with decades of precedent, the Court was speaking only of opposite-sex marriages.

He also acknowledged that in cases involving discrimination on the basis of race and sex, the Supreme Court has gone well beyond rational basis review. When a law targets a “discrete and insular class,” subject to pervasive prejudice and suffering from political powerlessness, governments must come up with an unusually strong justification for it. In Sutton’s view, however, gays and lesbians hardly lack political power. On the contrary, they have often succeeded in the political process, especially in recent years.

The force of Sutton’s opinion lies in its plea for judicial humility and its insistence that in the face of evolving social norms, federal courts should be reluctant to offer a novel reading of the Constitution to accelerate the pace of change, or to favor one or another side. That is wise and correct in many areas in which Americans have recently tried (sometimes successfully) to invoke the Constitution for contemporary political ends -- to create welfare rights, to strike down restrictions on commercial advertising, to invalidate affirmative action programs.

But there is a weak link in Sutton’s argument, and it has to do with the limits of judicial humility.

As late as 1945, racial segregation was widely believed to be constitutionally acceptable. In 1954, the justices ruled otherwise. As late as 1970, the Supreme Court had raised no serious objection to laws that discriminate on the basis of sex. By 1976, the justices made it clear they would take a strong stand against such laws. The Supreme Court has not deferred to the political process in areas that include the right to vote, freedom of religion and access to contraception -- even in cases in which the text of the Constitution, read in light of the original understanding, does not support an aggressive judicial role.

While it's true that gays and lesbians do not lack political power, they have been, and continue to be, subject to pervasive prejudice and hostility. For that reason, federal courts should require discrimination against them to be supported by a strong justification -- not merely a rational basis. The ban on same-sex marriage lacks that justification.

Sutton is right to emphasize that, in a free society, it's much better if social change comes from democratic processes, not federal courts. The Supreme Court has been prudent to decline to rule on same-sex marriage -- and it might do well to find strategies to continue to delay a constitutional resolution (though Sutton's ruling, producing a division in the lower courts, makes that harder).

But before long, the day of reckoning will come. To date, Judge Sutton has provided the best argument against the constitutional claim on behalf of marriage equality. It is almost convincing -- but not quite.

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